Surface plasmon resonance occurs when radiant energy is “coupled” (or transferred) to electrons in a metal. The wavelength of light at which coupling occurs depends on the characteristics of the metal that is illuminated and the optical properties of the surrounding environment. When there is a match or resonance between the energy and wavenumber of the light photons and the electrons at the metal surface, a transfer of energy occurs. The coupling of light into a metal surface produces a plasmon (i.e., a group of excited electrons which behave like a single electrical entity). The plasmon, in turn, generates an electro-magnetic field that typically extends on the order of about 100 nanometers (nm) above and below the metal surface and oscillates with optical frequencies.
U.S. Pat. No. 5,841,143 has proposed a waveguide integrated fluorescence sensor that includes a corrugated dielectric-metal-dielectric thin film stack that is fabricated on a p-n junction. Fluorescent light of a single wavelength is selectively transmitted through the corrugated thin film stack and converted into an electronic signal by the p-n junction. Wavelength filtering is enabled by evanescent mode (or evanescent wave) cross-coupling across the metal film. Such cross-coupling includes cross-coupling between surface plasmons across the metal film and cross-coupling between surface plasmons and waveguide modes across the metal film.